1. Field of the Invention
This invention relates to a method of continuously operating a counter-flow washing column, and more particularly, to a method of continuously operating a processing system comprising a continuous polymerization zone, a counter-flow washing column and a heat tube for separating solid polymer particles.
2. Description of the Prior Art
In the manufacture of polymers on an industrial scale, the polymerization reaction, i.e. the polymerization of the monomers, is conventionally carried out in a diluent which does not dissolve the polymer so that the resulting polymer is produced in the form of a slurry of solid polymer particles and a diluent. The reaction is ordinarily carried out in such a diluent because the heat of polymerization can be easily removed and the polymer can be relatively easily separated from the unreacted monomers and the reaction medium. This process is particularly carried out in the polymerization of olefins, which reaction involves a large amount of polymerization heat.
In such polymerization systems, particularly in the polymerization of olefins, it is also known to wash the slurry in a counter-flow washing column in order to remove the residue of the catalyst used for the polymerization of olefins, which catalyst includes transition metals and organic metal compounds. In such olefin polymerization systems, there are used as diluents and washing liquids, liquids having a relatively low boiling point and the washed slurry is heated in a heat tube for the purpose of separating the slurry into solid polymer particles and to convert the diluent into a vapor. Such processes as described above are disclosed in such patents as U.S. Pat. Nos. 3,262,922, 3,644,583, 3,285,899 and 3,428,619.
The washing of the slurries in a counter-flow washing column by the reference processes, is carried out in each case by relying only upon the density differences between the solid polymer particles and the diluent, which means that the slurry in each of the reference processes can be effectively washed only if the amounts of liquid and slurry introduced into the counter-flow washing column through various ports and the amounts of liquid and slurry discharged therefrom are always constant; however, the washing cannot be effectively carried out if the discharged liquid and slurry are not always constant.
In an actual polymerization process, the production performance fluctuates due to the changes in the catalytic action, the molecular weight of the polymer and the ratio of the monomers in, for example, a copolymerization reaction. This means that the amount of slurry introduced into the counter-flow washing column and the ratio of the solid polymer particles and diluent fluctuates as the aforementioned parameters change. This fluctuation according to the prior art methods destroys the balance between the washing liquid and the solid polymer particles in the counter-flow washing column and causes decreased washing efficiency in the counter-flow washing column which may adversely affect the quality of the solid polymer particles. Further, as the case may be, solid polymer particles may be entrained with the washing liquid to be extracted from the upper portion of the counter-flow wasing column and may affect the step of recovering the washing liquid. Also, the concentration of the washed slurry to be extracted from the lower portion of the counter-flow washing column may be changed and, as the case may be, such change may make extraction by this method nonfeasible. Thus, the prior art methods have encountered many difficulties.